Silver halide emulsions which can be used in a high sensitive silver halide light-sensitive material having sensitivity which can be used for the usual camera usually comprise silver iodobromide grains having a face-centered cubic crystal in which a solid solution is formed by silver iodide. A limiting amount of silver iodide having a face-centered cubic crystal in which a solid solution is formed is not more than 45 mol %. Usually, however, industrially important silver iodobromide is often used in such a manner that the silver iodide content is in the range of from 0.1 to 20 mol %.
As industrially important silver iodobromide emulsion grains, those in forms such as cubic, octahedral, dodecahedral, spherical and tabular are known. It is also known that tabular grains have a high covering force and further have high graininess, sharpness and color sensitization efficiency due to sensitization dyes. This is described in detail in Research Disclosure, Vol. 225, Item 22534, pp. 20-58 (January, 1983).
Tabular silver iodobromide emulsions have been prepared mainly by a method in which soluble silver salts such as silver nitrate are added to a solution containing bromide salts and iodide salts, i.e., the so-called single jet method as described in A. P. H. Trivelli & W. F. Smith, Photographic Journal, Vol. 80, p. 285 (1940) and Duffin, Photographic Emulsion Chemistry, The Focal Press, New York, pp. 66-72 (1966). However, the tabular silver iodobromide grains prepared by the above method have a broad grain size distribution and light-sensitive materials prepared using such tabular silver iodobromide grains are low in contrast. Thus, such light-sensitive materials are limited in their use, therefore, they are usually used as only negative light-sensitive materials for cameras. Furthermore, in accordance with the above method, the intragrain and intergrain silver iodide content distributions are broad and it is very difficult to control these silver iodide content distributions. As described in Japanese Patent Application (OPI) No. 99433/84 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"), the distribution of silver iodide in the inside of the grain not only has significant influences on anti-pressure characteristics, but also has definite influences on photographic sensitivity, gradation, rate of development and so forth. Thus, it is industrially very important to completely control the above silver iodide distribution. Accordingly, it is a disadvantage inherent to the single jet method that the grain size distribution and the silver iodide distribution in silver iodide grains and between silver iodide grains are substantially difficult to control.
To overcome the disadvantage of the single jet method that the grain size distribution is broad, U.S. Pat. No. 4,067,739 discloses a method in which a solution of bromide salt and silver iodide salt and a solution of silver nitrate are mixed in the presence of gelatin, the resulting mixture is subjected to physical ripening in the presence of a silver halide solvent to form seed crystals which are in the form of twinned crystals, and then silver nitrate and bromide salts are added under accelerated speed by the double jet method while maintaining the pBr at not less than 0.15 to thereby prepare tabular silver iodobromide grains having a narrow grain size distribution. In addition, U.S. Pat. Nos. 4,150,994, 4,184,877, 4,184,878, British Patent No. 1,570,581, West German Patent Application (OLS) Nos. 2,905,655 and 2,921,077 disclose a method in which silver halide containing at least 90 mol % of silver iodide is used as a seed crystal and, thereafter, silver iodobromide grains having a tabular twinned crystal form and having a narrow size distribution are formed by the double jet method. In accordance with this method, however, it is still difficult to control the silver iodide content distribution in the inside of and between the grains. Furthermore, since the tabular grains thus obtained are generally relatively thick grains, the resulting silver iodobromide emulsion is not an emulsion having a high aspect ratio which sufficiently exhibits the features of tabular grains as described above.
E. B. Gutoff, Photographic Science and Engineering, Vol. 14, page 248 (1970) discloses that a silver iodobromide emulsion having a high aspect ratio as defined in the present invention and Japanese Patent Application (OPI) No. 113928/83 can be obtained by introducing the solutions of bromide salt, iodide salt, silver salt and gelatin at the same time and then further continuously adding these solutions while maintaining the pBr at a constant value in the range of from 0.49 to 1.43. This method, however, has a fundamental problem that since it employs a continuous steady state method in which the emulsion formed is continuously withdrawn corresponding to the speed of addition of the solute and since nuclei are constantly being formed due to the method and are intermingled with finely divided tabular grains, there is an increase in the grain size distribution. Furthermore, since the distribution of silver iodide in the tabular silver iodobromide obtained in the inside of and between the grains is naturally uniform, it is impossible to control at will the silver iodide content at a specified position in the inside of the grains. Thus, the emulsion thus obtained is usually high in pressure fog, as described in Japanese Patent Application (OPI) No. 99433/84.
As a method of preparing silver iodobromide having a high aspect ratio, Japanese Patent Application (OPI) No. 113928/83 discloses a method in which the pBr of a dispersion medium in a reactor prior to the introduction of iodide salt is controlled to 0.6 to 1.6, the reactor prior to the introduction of silver salt and bromide salt is maintained in such a state as not to contain soluble iodide, and the pBr in the reactor is maintained at least at 0.6 during the process of introducing iodide. A similar method is disclosed in European Patent No. 84,637A.sub.2. U.S. Pat. No. 4,414,310 discloses a method for preparing a silver iodobromide emulsion having a high aspect ratio by using as seed crystals high iodine content silver halide grains (silver iodide content of not less than 90 mol %) which belong to the hexagonal crystal system and which have a diameter of not more than 0.1 .mu.m, and adding the silver salt and bromide salt solutions by the double jet method. In these methods, it is an important condition that substantially no soluble iodide salt is incorporated in the reactor prior to the introduction of the silver salt and bromide salt. In connection with the reason for that, Japanese Patent Application (OPI) No. 113928/83 describes that if iodide ions are previously dissolved in the solution phase, the silver iodobromide tabular grains formed are relatively thick and furthermore the number of non-tabular grains is increased to obtain a low aspect ratio. In connection with this, a detailed discussion appears in Trivelli & Smith, Photographic Journal, Vol. 80, pp. 285-288 (1940) which is referred to in Japanese Patent Application (OPI) No. 113928/83 and U.S. Pat. No. 4,414,310.
Therefore, it is generally considered that it is an essential condition to minimize the dissolution of iodide ions in the dispersion medium prior to the introduction of silver salt and bromide salt in order to obtain silver iodobromide grains having a high aspect ratio.
It can be generally said that a method for preparing a silver iodobromide emulsion having a high aspect ratio by the double jet method as described in Japanese Patent Application (OPI) No. 113928/83 and European Patent No. 84,637A.sub.2 has advantages in that the distribution of silver iodide in the inside of the grains can be relatively easily controlled, and further as compared with the single jet method and so forth, the grain size distribution can be made narrow to a certain extent. However, in the case where a high iodine content silver halide having a hexagonal crystal is used as a seed crystal, as described in U.S. Pat. No. 4,414,310, since, in a similar manner as in U.S. Pat. No. 4,150,994, the iodine distribution in the inside of the grain and between the grains basically depends on the balance between the dissolution of the high iodine content seed crystals and the speed of addition of silver salt and bromide salt, it is generally difficult to control the iodine distribution. It is therefore desirable that silver iodide is introduced directly in a solid solution form in the silver iodobromide grain from the beginning by using a known amount of soluble iodide salt to thereby control the silver iodide distribution in the inside of the grain.
In preparing a tabular silver iodobromide emulsion having a high aspect ratio by the double jet method, a practical problem is encountered that when the same emulsion is repeatedly produced, the reproducibility is generally very poor, as compared with the conventional emulsions, at the time of transferring from a small scape production to a large scale and when trying to reproduce at the same scale. This problem is assumed to be the result of fact that since the possibility of forming a twinned crystals along with a normal crystals during the formation of nuclei is greatly influenced by a slight change in pBr, then a slight change in the balance between the stirring condition and the speeds of addition of silver nitrate and silver bromide has substantial influences on the iodide ion concentration and the concentration distribution in the reactor. It is therefore desired to develop a method to decrease a large change in the probability of forming a twinned crystals due to a change in pBr during the introduction of the silver salt and bromide salt.
A high contrast is needed in an X-ray light-sensitive material, a positive light-sensitive material and a reversal light-sensitive material. In general, however, tabular grains having a high aspect ratio as produced even by the double jet method have a broad size distribution and thus a light-sensitive material having a high contrast is difficult to produce. Furthermore, if the size distribution is broad, the graininess is seriously reduced by large sized grains. Therefore, in preparing light-sensitive materials as described above, it is desirable for seed grains of a tabular double twinned crystals prior to the start of growth to be divided as finely as possible and to have a narrow size distribution. The reason for this, as described in Sugimoto, Photographic Science and Engineering, Vol. 28, pp. 137-145 (1984), is that changes in the size distribution of tabular grains having a high aspect ratio after the growth can be controlled to be smaller level when the change in the average size and distribution of seed grains is smaller. Furthermore, if the grain size of the seed grain is small, the grain size after the growth can be freely chosen. Of course, as described in J. S. Wey & R. W. Strong, Photographic Science and Engineering, Vol. 21, page 248 (1977), if the degree of supersaturation is decreased by lowering the speed of addition of a solute at the time of growth, the grain size distribution of the grown grain is broadened. If a light-sensitive material having a low contrast, in particular, is desired, the grain size distribution of the grown grain can be free to control. Thus, such tabular seed grains have a high degree of freedom in the manner in which they are used to produce grown grains.
As a result of extensive investigations, the present inventors have succeeded in developing a method for preparing a tabular silver iodobromide emulsion having a high aspect ratio which satisfies the above-described requirements.